A source of benzene and xylenes is catalytic reformate, which is prepared by contacting a mixture of petroleum naphtha and hydrogen with a strong hydrogenation/dehydrogenation catalyst, such as platinum, on a moderately acidic support, such as a halogen-treated alumina. Usually, a C6 to C8 fraction is separated from the reformate and extracted with a solvent selective for aromatics or aliphatics to produce a mixture of aromatic compounds that is relatively free of aliphatics. This mixture of aromatic compounds usually contains BTX, along with ethylbenzene.
Refineries have also focused on the production of benzene and xylenes by transalkylation of lower value C8+ aromatics with benzene or toluene to produce xylenes as increasingly important process. Chemical plants would ideally like to process as much of the heavy C8+ aromatics as possible while minimizing and potentially removing the toluene/benzene co-feed. Both transalkylation activity and dealkylation activity are important for a successful catalyst system. Transalkylation is the ability to transalkylate methyl groups to form xylenes. Dealkylation activity is the ability to dealkylate ethyl and propyl groups present on the C8+ aromatics to allow the formation of lower methyl/ring species that may transalkylate with higher methyl/ring species to form xylenes. Metal function is required to saturate olefins formed during dealkylation while maintaining the integrity of the aromatic saturations. As plants move to increased amounts of C8+ in the feed, acceptable activity and catalyst life become challenging.
Two zeolites may be combined in the same catalyst particle through means of physical mixing the two zeolites together, often with an alumina or other binder. In U.S. Publication 2016/0220987 (2015EM282), one or more catalyst compositions are disclosed which comprise a first zeolite having a constraint index of 3 to 12, a second zeolite comprising a mordenite zeolite synthesized from TEA or MTEA, at least one first metal of Group 10 of the IUPAC Periodic Table, and at least one second metal of Group 11 to 15 of the IUPAC Periodic Table. The mordenite zeolite is a high activity mordenite zeolite having a small particle size, referred to as meso-mordenite, as defined herein. These catalyst compositions may be used in a process for the conversion of a feedstock containing C8+ aromatic hydrocarbons to produce light aromatic products, comprising benzene, toluene and xylene.
Even with these advances in catalyst technology, a need exists for improved catalyst performance in aprocess for the conversion of C8+ aromatic hydrocarbons.